A U.S. Marine Corps MV-22 lands at a remote location near the Iraqi-Syrian border to resupply Marines and soldiers with the 3rd Cavalry Regiment, June 23, 2018. Iraqi Security Forces and coalition partners provided fire support to assist the Syrian Democratic Forces as they continue Operation Roundup, the military offensive to accelerate the defeat of ISIS remnants in the Middle Euphrates River Valley and Iraq-Syria border region.

Syrian Democratic Forces launched the third phase of ground operations to clear Islamic State fighters from northeastern Syria on Monday, following cross-border air and artillery strikes from the U.S.-led coalition in Iraq.

The operations aim to clear ISIS from areas along the Middle Euphrates River Valley and toward the border with Iraq, the Operation Inherent Resolve coalition said in a statement Tuesday.

Dubbed Operation Roundup, the effort began in May with the clearing of ISIS from near Baghuz, a village on the eastern bank of the Euphrates River. A second phase that ended in late July swept the militants from around the town of Dashisha.

For more than a month, the coalition has conducted operations in the village of Hajin and the countryside outside the city of Deir Ezzor to prepare for the advance. Coalition aircraft and artillery have carried out dozens of strikes in the border region in recent weeks.

The Syrian Democratic Forces, or SDF, said in a separate statement on Tuesday that the four-axis ground assault would clear ISIS from villages and farms in an area of about 22-by-6 miles. The troops would take measures to avoid civilian suffering, such as providing humanitarian crossings for fleeing civilians and emergency camps with medical services, the SDF said.

“While reaffirming our determination to move forward to end terrorism in the east of the Euphrates, (we) will spare no effort to do everything we can with regard to our humanitarian obligations,” the SDF statement said.

The U.S. has drawn criticism for partnering with the group, which is largely made up of the Kurdish People’s Protection Units, or YPG. Turkey considers the YPG an extension of the Kurdistan Workers’ Party, or PKK. Ankara and the U.S. State Department, among others, consider the PKK a terrorist organization.

Observers have also criticized the SDF for drawing minors into its ranks. Last week, the group’s top commander issued orders to all factions and formations prohibiting the recruitment of children under age 18.

In its statement on the new phase of the offensive, the coalition touted the Kurdish-led SDF as reliable and effective combat soldiers who abide by the laws of armed conflict. U.S. Army Maj. Gen. Patrick Roberson, coalition special operations task force commander, pledged continuing support to its partners and thanked them for their sacrifices in the anti-ISIS fight.

Syria’s Kurds control much of the country’s northeast, which includes the majority Kurdish region they call Rojava.

The Syrian regime has focused on fighting elsewhere during its seven-year-old civil war, but that could soon change as President Bashar al-Assad closes in on the final rebel enclave in the country’s northwest.

Russia has also reportedly warned the U.S. more than once in recent weeks that it is prepared to back an Assad regime offensive in eastern Syria to clear militants from within a 34-mile “deconfliction zone” surrounding a small garrison near the city of At Tanf, where U.S. forces are based.

Late last week, a company-sized element of Marines with the U.S.-led coalition carried out an exercise at the base, which observers saw as a show of force to ward off any impending incursion. The exercise involved an aerial assault and live-fire rehearsal, officials said, and Russia was notified of the event through existing deconfliction processes ahead of time.

Coalition advisers turn their M777 Howitzer to prepare for a new fire mission at an outpost near Dashisha, Syria, June 9, 2018. The coalition has provided assistance and training to the Syrian Democratic Forces as they continue to reclaim their homeland from ISIS during Operation Roundup.TIMOTHY R. KOSTER/U.S. ARMY